It is known that imparting acoustic frequency vibrations to the human skull, either directly or via teeth, results in improved hearing in certain hearing impaired individuals. Hearing aids and assistive listening devices taking advantage of this phenomenon generally include a microphone for transducing ambient acoustic energy into an electrical signal, an audio amplifier, a transducer for converting the amplified audio signal to mechanical vibrations, and some mechanism for imparting the vibrations to a tooth or to bone structure in the skull. The imparted vibrations stimulate the cochlea, resulting in a perception of sound. Examples of such devices are disclosed in U.S. Pat. No. 5,460,593 to Mersky et al., and U.S. Pat. No. 5,033,999 to Mersky, the disclosures of which are both incorporated herein by reference.
Some intra-oral hearing aid devices provide for a bracket that is bonded on one surface of a tooth. An in-mouth housing including an actuator and electronic components is configured to engage a side of a tooth that has not been anatomically modified. The bracket may retain the housing, but primarily is intended to pass vibrations from the actuator to the skull. However, such devices suffer from various drawbacks. In particular, if the bracket de-bonds from the tooth, then vibrations are no longer passed from the actuator to the skull, rendering the device inoperable. Moreover, de-bonding of the bracket may result in insufficient retention of the housing, such that the housing falls out of place and/or is rendered inoperable. In addition, such devices provide for the vibrations from the actuator to be conducted through the bracket. The actuator therefore stand-offs or is inefficiently spaced from the tooth, and consequently projects into the cheek (e.g., at least by the thickness or depth of the bracket and bonding material, which is typically at least about 0.080 inch). This stand-off dimension or spacing may result in cheek discomfort and/or externally visible facial puffiness.
Another problem associated with many prior devices relates to the inadequate or ineffectiveness of the vibrator in accurately transducing the applied electrical signals into mechanical vibrations. Another problem associated with prior devices relates to the ineffective or inefficient manner in which the transducer is coupled to the hard bone tissue. In particular, many prior art tooth coupling techniques suffer from various disadvantages, including; low coupling efficiency (e.g., resulting in a significant loss of mechanical energy); deterioration of coupling efficiency over time; difficulty of removing or replacing the vibrating member; or a combination of these disadvantages. For example, in human experiments where the actuator was part of a C-ring that went behind the last maxillary molar, the inventor found that over time, the “spring force of the C-ring” became unsatisfactorily weak, and thus the coupling efficiency was reduced.
Further, many prior devices provide vibrating members that rely on an osseointegration member to secure the devices to bone tissue and to act as the skull stimulation site. Such devices involve a major surgical procedure, and have longer-term problems associated with the surgical implant.
Other prior art devices secure the actuator to the skull by magnetic force associated with an osseointegrated implant. Such deliver electromagnetic signals transcutaneously to the implanted member which then vibrates. This transcutaneous, as opposed to direct, coupling of the signal to the implant can result in a considerable loss of energy particularly when the scalp tissue swells. This energy loss increases with the square of the distance between the external unit and implant. Moreover, the magnetic attraction between the external unit and vibrator will deteriorate over time resulting in further loss of efficiency. Finally as a practical manner, should the implanted member need removal, a permanent hole will remain in the skull bone.
Most present systems that impart vibrations to bone tissue rely on magnetic or piezoelectric transducers. Magnetic transducers involve reciprocating translation of a magnetically permeable disk and armature member. These devices tend to be inefficient in transducing electrical energy into reciprocating translatory motion, and are operable only over limited frequency ranges due to inertial constraints of the movable member. Piezo-ceramic devices also tend to be inefficient, given they require relatively high voltages and are notoriously ineffective at frequency ranges below 1 KHz.